In response to the RFA-NS-16-021 to address some of the gaps in our knowledge of the biologic mechanisms of the commonly occurring cerebrovascular disease and age-related white matter (WM) disease at the molecular, cellular and brain circuit level, we propose to study the role of pericytes in WM disease using new pericyte-specific animal models and advanced molecular, cellular and neuroimaging methods, and circuit level analysis. Up to 45% of all dementias worldwide are estimated to be wholly or partly due to age-related small vessel disease (SVD) of the brain. Pericytes are vascular mural cells embedded in the wall of small blood vessels such as capillaries, pre-capillary arterioles, and post-capillary venules. In the brain, they control key neurovascular functions such as blood-brain barrier (BBB) integrity and cerebral blood flow (CBF). Pericyte degeneration leads to BBB breakdown and impaired hemodynamic responses, and is found in neurologic disorders exhibiting SVD, WM disease and cognitive impairment such as Alzheimer's, stroke and CADASIL - the most common genetic cause of ischemic SVD and VCID; yet, the role of pericytes in the pathophysiology of SVD and WM disease is largely not known. Based on our pilot data obtained in pericyte-deficient platelet- derived growth factor receptor-? (Pdgfr?F7/F7) mice and published studies in TgNotch3R169C mice expressing Notch3 CADASIL mutant in vascular smooth muscle cells and pericytes, we hypothesize that WM pericyte loss leads to BBB breakdown and CBF reductions causing loss of oligodendrocytes, demyelination, axon damage, disrupted connectivity and disintegration of CNS circuits, which leads to functional deficits and neuron loss. Since currently available pericyte-deficient Pdgfb/Pdgfr??lines and TgNotch3R169C mice are not pericyte specific, to test our hypothesis we have generated new pericyte-specific lines such as Pdgfr?-Flp; Cspg4-FSF-CreER; iDTR mice with inducible pericyte ablation, and will develop a new mouse line with inducible expression of Notch3R169C mutation only in pericytes. We will use i) cutting-edge longitudinal dynamic contrast-enhanced magnetic resonance imaging (MRI) of regional BBB integrity, dynamic susceptibility contrast MRI of CBF, diffusion tensor imaging (DTI) and DTI-based tractography for structural/connectivity changes, and tract-tracing based connectomics for CNS circuit level analysis; ii) behavior tests; and iii) immunohistology, neuropathology, flow cytometry, and electron microscopy tissue analyses. We will determine the effects of global inducible pericyte ablation (20-70%) (AIM 1), focal inducible pericyte loss in the anterior cingulum of the corticolimbic ciruit (AIM 2) and pericyte-specific inducible Notch3R169C expression (AIM 3) on BBB integrity, CBF reductions, WM integrity, disruption of CNS circuits and functional deficits (behavior). We expect that the proposed studies will contribute towards better understanding of the mechanistic basis of WM disease in vascular cognitive impairment and dementia, and will establish pericyte as a new key target for WM disease.